Colour printing



J. F. cRosFlELD ET AL yCOLOUR PRINTING July 8, 1958 Filed Aug. 25. 1954- 2 Sheets-Sheet l l m V Y *9.)

u) m wl* l 'Q 1 U (9 a sa L1 u 'i Ril Il \f N u an 9 9 \O 'v' LL l I V V N u M al Pel al l u -a ml m al @Inventors By @4,414 03. M Attorney July 8, 1958 J. F. cRosFxELD ET AL 2,842,610

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Attorney United States PatentO COLOUR PRINTING `lohn Fothergill Crosfield and Gordon Stanley James Allen, London, England, assignors to J. F. Croslield Limited, London, England, a British company Application August 23, 1954, Serial No. 451,466

Claims priority, application Great Britain August 25, 1953V 12 Claims. (Cl. 178-5.2)

This invention relates to the production of separation negatives or positives for use in colour reproduction, and more particularly to colour correction in such separation prints.

For colour printing, an original is generally photographed through a number of colour filters to produce colour separation negatives, and these are used to prepare printing cylinders or plates for the different colours to make multi-colour prints. However, it is known that the separation negatives, or the positives made from them, are not usually suitable for the preparation of printing surfaces directly, owing to the difficulties of obtaining colour filters and inks of suitable complementary colour response. For example, a printing ink which is nominal- -1y magenta may also reflect some light in the cyan part of the spectrum, so that, where both inks are printed at a given element of the picture, the weight of cyan ink applied should be reduced by an amount dependent on the amount of magenta ink applied.

Accordingly, it -is known to correct each element of the colour separation positive or negative of each colour in accordance with the values of each of the other colours present in that element. For example, in a three-colour process, the uncorrected negative of each colour may be exposed through a pair of superimposed masks formed by partial exposure of negatives of the other two colours, to produce a corrected separation positive of that colour. In this photographic masking method, the degrees of partial exposure required to provide the correct degree of masking are difficult to control and results tend to be largely a matter of trial and error, and take a long tune. f

It has also been proposed to scan the uncorrected separation negatives in register simultaneously with photoelectric cells and then mask each of the electric output signals from these cells in accordance with the others. Correctedv prints are then prepared by exposure to a scanning light source, the intensity of which is varied in `accordance with the corrected electrical output signal. Considerable difficulties arise in this electrical method in maintaining register between the light source scanning the uncorrected prints and the light source exposing the corrected print, and also between the ultima-te corrected prints of the different colours.

Instead of scanning the uncorrected negatives to produce a corrected print, it is also possible to scan the original, `where it is a transparency, with photo-electric cells through colour filters, thus obtaining theguncorrected signals directly in electrical form, and then modifying them with respect to one another, as described above.v Again, it has been proposed to scan the transparency with a flying spot from a llight source, an image of the spot transmitted through the transparency and through a main colour tilter'being used to expose a colour separation negative, correction being obtained by controlling the intensity of the light spot in accordance with the electrical for feeding into the computer.

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2 spot .also through the transparency and through filters of the other colours. v

According to the present invention, in a multi-colour printing process, each of the required colour separation prints is formed by direct exposure to a scanning spot of light, without the interposition of a mask or transparency in the light path from the source to the print to be exposed, and the intensity of the light spot is con trolled, element by element during scanning, by the output of an electrical 'computor which receives information from photo-electric cells receiving light of a number of colours passing through a transparency or through uncorrected separation prints, and originating in the same scanning spot. The computer performs the necessary correction of the spot intensity to modify the exposure of each element ofthe print with respect to the values of the other colours present in the transparency or in the corresponding element of each of the uncorrected separation prints. Where a transparency is used as the original, the corrected separation negative may be partially exposed by a scanning spot of light of constant intensity passed through the original and through a colour filter, being scanned a second time in the manner according to the invention, either subsequently or beforehand, in order to obtain the correction.

The scanning spot of light from the light source is, in general, in the method according to the invention, split `into two paths, by means for instance of a semisilvered mirror. One path leads to the print to be prepared, while the other is used to derive the colour signals` As the light exposing the print does not pass through the transparency or un. corrected negatives, this method makes it possible to expose the print fully for'those colour plates where the transparency is black. That is to say, it facilitates the v elimination of colour from the colour plates where' the of light of the colours to be used, each beam beingA outputs of photo-electric cells receiving light from the g focussed on the cathode of a photo-electric cell. The electrical outputs of the photo-electric cells are amplified and fed into the computer to provide the information for controlling the intensity of the source. p

If the originals are uncorrected negatives, then the second path is split by means, for example, of partsilvered mirrors, into a number of beams equal to the number of uncorrected negatives, each beam being passed through an uncorrected negative onto the cathode of a` photo-electric cell, which is used to feed the computer as before. The uncorrected negatives must, of course, be in register, so that the scanning spot passes through corresponding elements in each negative simultaneously. The output of the computer controlling the intensity of the scanning spot may be used also to control means to render the input information tothe computer substantially independent of the instantaneous brightness of the scanning spot, so that it is dependent only on the relative densities of the diiferentcolours in the transparency or uncorrected prints. For example, the output of the computer may control in a reciprocal manner the gains of amplifiers feeding the different signals from the photo-electric cells into the computer.

Alternatively, the signals from the photo-electric cells may be divided by this output -of the computer in a logarithmic dividing circuit before being passed on to the computer. Instead of using the output of the computer, a further photo-electric cell could be used to derive a signal proportional to spot intensity and feed the amplifiers or 3 dividing circuit in opposition to the signals, from` the main photo-electric cells in rthe sarne way.

The scanning spot of light may be obtained in various Ways, for example,k from a mechanical system involving a rotating mirror drum. In such a system, a small lamp of` high luminous intensity is placedA at the focus of alens which` forms a beam of parallel rays. This beam falls on afrotating mirror drum and is reflected through a second lens, to bring it to a focus. to form a spot image. In this way the spot is caused to scan repeatedly along a straight line. If the lamp is now moved slowly in a direction parallel with the axis of the drum, spot image is caused to produce a complete raster scanning a rectangular area.

However, the most convenient source for the scanning spot is the screen of a flat faced cathode-ray tube, on which a complete raster is produced by means of the usual` electrostatic or electromagnetic deflection arrangements. This has the advantage of virtual lack of inertia. An example of apparatus according to the invention for use in a four-colour printing process will now be describedwith reference to the accompanying drawings, in which:

Figure 1 is a general schematic diagram ofthe optical system and electrical connections of* that form of the apparatus for use with transparencies; and

Figure 2 shows the modified optical systemfor use with uncorrected separation negatives or positives.

Referring rst to Figure 1, a cathode ray tube 1 has a at face 2, on which is produced a rectangular raster 3. The usual focussing coil 4 is driven from a suitable circuit 5, and the deflection of the electron beam to produce the raster is obtained by electromagnetic deflection coils 6, driven from horizontal and vertical time base circuits indicated by the block 7. The phosphor on the face 2 of the tube is of such a kind as to give effectivelyl white light covering as equally as possible all the wavelengths of the Visible spectrum. The brightness of` the'raster may becontrolled in a well known manner byalteration ofthe potential of the grid 8. with respectto the cathode 9. It is, in fact continually varied by the application to the grid of a high frequency. alternating currentl signal from an oscillator V10, so, that, as theelectron beam travels across the face 2 of. the. tube, the spot oflight produced varies in. intensity at ahigh frequency. This means that the subsequent electrical circuits, to be described, which handle the outputs from photo-electric cells, have to deal with an alternating current signal, which considerably simplifies their design.

An image ofthe spot on the screen of the cathode-ray tube 1 is produced bya lens 11on the colour transparency shown at 12, and theraster is of such an amplitude that it just scans the wholeV of the transparency. Between the tube 1 and the lens 11 isa partially silvered mirrorV 13 which diverts someof the light-throughalens 14 onto a plate 15 on which is the correctedA negative or positive to beformed.

Light from the spot transmitted by the transparency 12A is collected by a lens 16 and divided into three paths by partially silvered mirrors 17 and 18, so asto fall -on each of three photo-electric multiplier cells 19, 20. and 21. In front of each cell is a lter 22, say red, blue and green in front of cells 19, 20, 21, respectively. If dichroic mirrors were used instead of partially silvered ones at 17 and 18, the filters could, of course, be dispensed with.

The electrical outputs of the cells 19,120 and 21 are fed to cathode follower valve amplifiers 24, whose characteristic is that they have a high inputimpcdance and low -output impedance. The output of each cathode follower circuit is proportional to the transmissionfactor of the transparency for the particular colour selectedby the lter in front of the corresponding cell, multiplied by the instantaneous brightness of the scanning spot.

The output signal from the cathode follower connected to cell 19 is proportional to the transmission factor of the transparency for red rays, and is torbe used vforpro-` ducing the printer of the complementary colour, namely,

cyan. This will therefore be known as the cyan signal channel. Similarly, the channel from cell 20 is the yellow printer channel, and from cell 21 the magenta printer channel.

Also receiving light direct from the scanning raster is a photo-electric multiplier cell 25 feeding a cathode follower circuit 26. Now t-he output of this circuit is proportional simply to the instantaneous brightness of the scanning spot. Its function is to provide a signal which, when divided into the signal from each of the other three cathode follower circuits, leaves a signal of magnitude dependent solely on the properties of the element of the transparency being scanned, and independent of spot brightness.

The division is performed logarithmically in the three mixing circuits 28M, ZSY, and 28C, in the magenta, yellow and cyan signal channels respectively. The signals in the colour channels and in the channel from the cell 25 are each fed to circuits which produce outputs proportional to their` logarithms, these circuits being indicated by t-he blocks 27M, 27Y, 27C and 29 respectively. For example, each of these circuits may comprise a high resistance in series with a germanium crystal rectifier. The inputvoltage is applied across the pair in series, so that the current through Iboth is substantially proportional to the input signal. Then the voltage developed across the rectifier will be substantially proportional to the logarithm of the input.

The logarithm of the ouput of cathode follower 29 is subtracted from that in each of the three colour channels in the mixing circuits 28M, ZSY and 2SC. This may be done very simply in a T resistance network, the output representing the difference being taken from the common resistance forming the centre stem of the T.

Now the input to each mixing circuit is proportional to the sum of the logarithms yof the transmission factor for the particular colour and of t-he spot brightness. Output is therefore solely proportional to the-logarithm of the transmission factor. But the inverse density at any point in the transparency for a particular colour is itselfproportional to the logarithm of the transmission factor. So the output of each of the three mixing circuits 28M, 28Y and 28C Vis proportional to the inverse density of the transparency in respect of the colour of the corresponding filter 22, and hence proportional to the density of the complementary colour, i. e. the colour of the printer to be prepared from that channel.

Masking of each colour signal with respect to the other two is performed in the masking circuits 29M, 29Y and 29C. Each of these circuits receives a signal from its ownV `channel and also from the other two channels throughtwo of the three inverting and attenuating circuits 30M, 30Y and 30C. Each of the latter circuits inverts the signalrin itschannel to make it of opposite sign, and then attenuates it to two different levels, to be fed toV each'of the other two colourchannel masking circuits.A For example, the circuit 30Y feeds a signal proportional to the yellow intensity to each of the masking circuits 29Mv and 29C in aV pre-determined proportion, tol reduce the signals in eachl of those channels by an amountV proportional to the, yellow signal. The levels at which these masking signals are `fed into the other channels` are adjustable, and depend on the particular inks and colour lters used.

The corrected output signals in the three channels, magenta, yellow and cyan, are then passed to a circuit shown. in Figure 1 diagrammatically by the block 31. Thefunction of this circuit 31, is to derive a fourth signal for the preparation of a,black printer. Where there isa black element in the original, this will, in a threecolour, process, call for the printing of a heavy weight of allthree colours, and in fact many ofthe elements of the original will call for the printing of all three colours to someextent. Now heavy weights of all three colours may equally'. well ,bereplaced by the single-colour black, resulting in a considerable saving in the/expensive coloured inks', as well as giving improved4 reproduction. Accordingly, in the four-colour process, whenever possible, black should be used and the weights of the other three colours reduced, by the amount of the black printed, since black effectively replaces a combination of all three colours in a subtractive process.

'Ihe circuit 31 may achieve this in one of a number of Ways. For example, the three inputs from the colour channels could be fed to a conventional amplitude selection circuit, such as the anodes of three thermionic vdiode valve rectifiers, the cathodes of which are connected to a common load resistance. It is arranged that the signal of smallest amplitude of the three is the most positive. Then that signal will pass through its corresponding diode, raising the cathode potential and cutting off the other two. The output signal for the fourth, or black printer is taken from the potential appearing across the load resistance, and each of the signals in the remaining channels is passed on reduced by this amount. Thus, when signals are present in all three channels at the input, the smallest will be suppressed, the black will be passed on at this amplitude, and the other two reduced by part of the amount of the black signal.

Each of the signals is modified in two further ways before being applied to the grid 8 of the cathode ray tube 1, to control the spot intensity. This modification is carried out in circuits 32 and 33 respectively in each of the four channels. In circuit 32 the density range of the print is altered to emphasise the highlights and shadows, by reducing the amplitude of the signal over its middle range of amplitudes. This is performed by passing the signal through a network of suitable non-linear` characteristics. A similar non-linear circuit 33 in each channel modifies the signal to allow for the non-linear response of the cathode ray tube and of the phosphor in the different spectral regions, and arranges that the brightness of each spectral component of the spot is substantially linearly dependent of the output signal of the corresponding stage 32.

By means of a switch 34, the grid 8 of the cathode ray tube 1 can be connected at will to any one of the four output signal channels, and in fact each is connected in turn while a complete scanning operation is performed to prepare a corrected separation print placed at 15. From these four corrected prints the printing cylinders may be prepared.

By means of a phase-reversing process applied to the signals at any stage in the system, it will be possible to produce corrected positives instead of corrected negatives at 15, if desired.

The cells and computer have an extremely rapid response which, for the sake of clearness, may be regarded as instantaneous, and, therefore, the analysis and correction are effectively carried out while the scanning spot is stationary. The correcting action described therefore takes place in each element of the reproduction successively.

In a modified form of the above system, the negative to be prepared is scanned twice. It is scanned once through the transparency 12 and through a filter of the appropriate colour, the spot of light on the screen of the cathode ray tube being of constant intensity throughout the scanning operation. Alternatively the raster could be replaced in this case by a simple large light source of uniform brightness. A second scanning operation is carried out by light from the spot or other light source without the interposition of a transparency or filter, While -simultaneously a second beam, divided from that scanning the negative to be prepared, scans the transparency yand falls on the cathodes of three photo-electric cells through colour filters as before, and the electrical outputs of these provide the information for colour correction, which is .fed into a computer to control the intensity Yof the light spot as before. In this modified system, therefore, the negative is at first partially exposed in an uncorrected form and thelcorrection is applied subsequently in a second scanning operation. As in the first example, signals from the photo-electric cells are divided by signals representing the spot intensity, in order to cancel the effect of variations of spot intensity in the input to the computer during this second scanning operation.

An important feature of this modification is that the definition of the corrected separation print is only limited by the definition of the optical system used in the first scanning to form an image of the transparency 12 on the print to be prepared, even though the definition of the correction is only as high as that of the scanning raster 3. Furthermore, some of the colour signal is transferred by optical means from the transparency to the negative to be prepared, so that the information passed through the computer represents only part of the required density range, not the whole signal.

In a system according to the invention for preparing colour corrected positives or negatives from uncorrected lseparation negatives, the rst arrangement described above must be further modified. Such a system is required where the original is not a transparency but is fiat copy such as artists sketches, from which uncorrected separation negatives may be prepared in the normal way by photography through colour filters.

The optical part of such a system is shown in Figure 2, the remainder being the same as Figure l. Here the light from the raster 3 falls as before on the print 15 to be prepared, by partial reflection by mirror 13. But the beam passing through the mirror 13 then is divided by partially silvered mirrors 40 and 41 into three beams, each of which falls on one of the three uncorrected separation negatives, which have produced by photography through red, blue and green filters and which are shown at 42, 43 .and 44 respectively. The light passing through these negatives is collected by lenses 45 yand falls on photo-electric cells 19, 20 and 21 as in the system of Figure l. It will be observed that no colour filters are used in this arrangement, but that the three negatives must be in exact register, so that the scanning spot of light falls on the corresponding element in each negative simultaneously.

A number of other modifications are possible in the systems described. For example, it will easily be seen that it may be made to suit a printing system using any number of colours.

The signal for cancelling spot brightness variations, instead of being taken from a photo-electric cell 25, could be taken directly from the signal fed to the grid 8 of the cathode ray tube. Again, instead of being divided logarithmically into the signals in the colour channels, it could be used to control reciprocally the gains of amplifiers through which the signals are fed, either by altering the screen grid potential of a valve, or by varying the grid bias of one or more variable mu stages, so that increase of spot brightness reduces the gain, and vice versa.

The invention has a number of substantial advantages. Thus, whilst the light source is permitted to vary in intensity in order to carry out the colour correction, this variation does not occur at the input to the computer from the colour analysing channels, due to the complementary variation in spot intensity and amplifier gain. This feature makes it possible to use a single scanning source for both analysis and reproduction and to carry out colour correction to the very high order required by the colour printing industry.

The use of a single source for analysis and reproduction furthermore, in addition to giving economy in apparatus, eliminates a host of possible sources of failure and distortion, in comparison with a system using two scanning spots, one for analysis and one for reproduction. Finally, the transparency and corrected negative or the uncorrected negatives and corrected positive are rigidly fixed in register quite independent of the scan- 7 ning raster, so that the latter may be rotated to scan the transparency or uncorrected negatives more than once and at a .number of different angles, to eliminate line effects on the reproduction.

We claim:

1. Apparatus for producing corrected colour separation prints from a colour transparency, comprising a light source of variable intensity, means for causing said source to scan over an area, a first image-forming means producing a first spot of light forming an image of said source on said transparency, means for dividing the iight beam of said irst spot passing through said transparency into a plurality of subsidiary beams, a light filter in the path of each of said subsidiary beams, a photo-electric cell in the path of each of said subsidiary beams, an electronic computer, said computer being electrically connected to said photo-electric cells, a second image-forming means producing a second spot of light forming an image of said variable intensity light source on a photosensitive surface, and means operated by said computer for controlling the brightness of said second spot.

2. Apparatus for producing corrected colour separation prints from a color transparency, comprising a light source of variable intensity, means for causing said source to scan over an area of said transparency, a first image-forming means producing a rst spot of light forming an image of said source on said transparency, means for dividing the light beam of said rst spot passing through said transparency into a plurality of subsidiary beams, a light filter in the path of each of said subsidiary beams, a photo-electric cell in the path of each of said subsidiary beams, an electronic computer, said computer being electrically connected to said photoelectric cells, a second image-forming means producing a second spot of light forming an image of said variable intensity light source on Ia photo-sensitive surface, and means operated by said computer for controlling the intensity of said light source.

3. Apparatus according to claim 2, wherein said light source is a fluorescent spot on the screen of a cathoderay tube having a grid, and said means for controlling the intensity of said source includes the grid of said cathode-ray tube.

4. Apparatus for producing corrected colour separation prints from a plurality of uncorrected separation prints, comprising a light source of variable intensity, a plurality of image-forming means, said image-forming means being arranged to form an image of said variable intensity light source on corresponding elements of each of said uncorrected prints, means for causing said source to scan over an area, a photo-electric cell in association with each of said uncorrected prints and adapted to receive light from said image of said source through said uncorrected print, an electronic computer, said computer being kelectrically connected to said photo-electric cells,

a further image-forming means forming a further image of said variable intensity light source on a photo-sensitive surface, and means operated by said computer for controlling the brightness of said further image.

5. Apparatus for producing corrected colour separation prints from a plurality of uncorrected separation prints, comprising a light source of variable intensity, a plurality of image-forming means, said image-forming means being arranged tokform an image of said variable intensity light source on corresponding elements of each of said uncorrected prints, means for causing said source to scan over an area, a photo-electric cell in association with .eachof said uncorrected prints and adapted to receive light from said image of said source through said uncorrected print, an Aelectronic computer, said computer being electrically connected to said photo-electric cells,

, 8 a `further image-forming means forming a further image of said variable intensity light source on a photo-sensitive surface, and means operated by said computer for controlling the intensity of said light source.

6. Apparatus according to claim 5, wherein said light source is a fluorescent spot on the screen of a cathoderay tube vhaving a grid, and said means for controlling the intensity of said source includes the grid of said cathode-ray tube.

7. Apparatus for producing corrected colour separation prints from a colour transparency, comprising a cathoderay tube, said cathode-ray tube having a fluorescent screen and a grid controlling the intensity of the spot formed on said screen, a first image-forming means forming an image of said spot on said transparency, means for dividing the light beam from said image passing through said transparency into a plurality of subsidiary beams, a light lter in the ypath of each of said subsidiary beams, a photoelectric cell in the path of each of said subsidiary beams, an electronic computer, said computer receiving electrical signals from each of said photo-electric cells, and producing a signal controlling the electric potential of said grid, a second image-forming means forming an image of said spot on a photo-sensitive surface, and scanning means acting on said cathode-ray tube for causing said spot to traverse an area over said screen.

8. Apparatus according to claim 7, comprising also means responsive to the intensity of said spot and modifying the signals fed from said photo-electric cells to said computer in a manner such as to render said signals substantially independent of said intensity.

9. Apparatus according to claim 7, comprising also means responsive to the electric potential of said grid and modifying the signals fed from said photo-electric cells to said .computer in a manner such as to render said signals substantially independent of said intensity.

10. Apparatus for producing corrected colour separation prints from a plurality of uncorrected colour separation prints, comprising a cathode-ray tube, said cathoderay tube having a fluorescent screen and a grid controlling the intensity of the luminous spot formed on said screen by the cathode ray, a plurality of image-forming means forming an image of said spot on corresponding elements of each of said uncorrected prints, a photo-electric cell associated with each uncorrected print and adapted to receive light from said image through said uncorrected print, an electronic computer fed with electric signals from each of said photo-electric cells and producing a signal controlling the electric potential of said grid, a further imageforming means forming an image of said spot on a photosensitive surface, and scanning means acting on said cathode-ray tube for causing said spot to traverse an area over said screen.

l1. Apparatus according to claim 10, comprising also means responsive'to the intensity or" said spot and modifying the signals fed from said photo-electric cells to said computer in a manner such as to render said signals substantially independent of said intensity.

12. Apparatus according to claim 10, comprising also means responsive to'the electric potential of said grid and modifying the signals fed from said photo-electric cells to said computer in a manner such as to render said signals substantially independent of said intensity.

References Cited in the le of this patent UNITED STATES PATENTS 2,434,651 Hardy Jan. 13, 1948 2,605,348 Hall July 29, 1952 2,691,696 Yule Oct. 12, 1954 2,740,828 Haynes ,Apr. 3, 1956 

